Catalytic carbonylation of aryl bromides and iodides to yield aldehydes is reported to take place with (P.phi..sub.3)PdX.sub.2 (.phi.=phenyl, X=halogen) or PdCl.sub.2 as catalysts under hydrogen pressure and a base, usually a tertiary amine (Schoenberg A. et al., (1974) J.Am.Chem.Soc., Vol. 96, p. 7761; Yoshida H. et al., Bull.Chem.Soc.Jpn. (1976) Vol. 49, p. 1681; Heck R. F., "Palladium Reagents in Organic Synthesis", Academic Press, New York, (1985) pp.359-361).
It is also possible to convert aryl bromides and iodides to aldehydes in the absence of dihydrogen with palladium P.phi..sub.3 complexes as catalysts, by using a hydrogen donor, such as a silyl hydride compound or a formate salt (I. Pri-Bar and O. Buchman, J.Org.Chem. (1984), Vol. 49, p. 4009).
However, the art teaches away from utilization of aryl chlorides in these processes, explicitly stating that these compounds are inert. Indeed, we have also confirmed, in our experiments, the inertness of chlorobenzene with palladium P.phi..sub.3 complexes, under conditions that are reported for converting bromo- and iodobenzene to benzaldehyde.
Chloroarene chromium tricarbonyl complexes react with CO and H.sub.2 in the presence of a base and Pd(P.phi..sub.3).sub.2 Cl.sub.2 as catalyst to yield the corresponding aldehyde (Mutin, R., et al.,(1988) J.Chem.Soc.,Chem.Commun., p.896). However, this reaction utilizes stoichiometric amounts of the chromium complex and it is explicitly stated that free aryl chlorides are not reactive.
A serious limitation of the above synthetic processes, which hinders industrial utilization, is the fact that aryl chlorides which are much more attractive as starting materials than aryl bromides and aryl iodides, are not reactive.
Recently, after the first filing of this patent application on Jul. 5, 1989, Huser et al. described the carbonylation of chlorobenzene with dihydrogen in the presence of the catalyst Pd(PR.sub.3).sub.2, wherein R is isopropyl or cyclohexyl (Huser, M. et al., Angew.Chem.Int.Ed.Engl. (1989) Vol. 28, p.1386).
We have discovered conditions under which it is possible, for the first time, to directly carbonylate aryl chlorides to aldehydes in the absence of hydrogen. When a catalytic amount of a chelating phosphine ligand is used in conjunction with a palladium catalyst, preferably in a polar solvent, aryl chlorides react with CO and a formate salt under mild pressure to yield the corresponding aromatic aldehyde.